1. Field of the Invention
The present invention relates to alignment control systems for ophthalmic instruments, and more particularly to an automatic alignment control for overriding manual alignment means of an ophthalmic instrument.
2. Description of the Prior Art
Many ophthalmic instruments require alignment of a test axis with the center of a patient's eye and an element of the instrument to be spaced a chosen distance from the eye. One such instrument, the non-contact tonometer, has been used by practitioners in their ophthalmic practice for more than 20 years.
The early non-contact tonometers, manufactured by American Optical Corporation, used a joystick, a handle extending from a ball mounted to provide pivotal motion about the center of the ball, in order to move the instrument in a horizontal (X-Z) plane. To move the test axis horizontally in the X direction toward the center of the patient's eye, the joystick is tilted in a corresponding direction. To move the element in the Z direction toward or away from the eye, the joystick is tilted in a corresponding direction. Obviously, compound horizontal movement can be obtained by moving the joystick in a direction between the aforementioned orthogonal directions.
Vertical Y-axis motion in these earlier non-contact tonometers was provided by a separate knob linked by a belt to a threaded shaft mounted for rotation on the carrier and engaging a nut on the support to provide the vertical movement of the test axis. The "firing" button used to actuate the air puff of the early non-contact tonometers was located in the center of the knob used to control vertical, Y-axis, movement.
Later models of the non-contact tonometers produced by Reichert-Jung Ophthalmic Instruments (a successor to American Optical Corporation) and Tokyo Optical Company (Topcon) combined the vertical Y-axis control and "firing" button into the joystick. An example of this type of joystick is taught by Japanese Publication No. 4-50562. Such controls have several disadvantages as taught by the above-mentioned Japanese publication. One problem encountered resulted from twisting the wires used to connect the "firing" button to the instrument electronics and the expense associated with eliminating the twisted wire problem. Another disadvantage of rotating the joystick to control vertical motion was that the practitioner could not accomplish vertical motion when grasping the joystick in the conventional manner because the vertical motion frequently required one or more revolutions of the joystick in order to vertically position the test axis in the center of the eye. Also, many practitioners encounter difficulty in maintaining a horizontal location while adjusting the vertical location of the instrument.
One system for determining the position of a test axis in relation to the center of the eye, as well as the distance of an element from the eye, is taught in commonly owned U.S. Pat. No. 4,881,807. This patent discloses an optical alignment system having camera means and a visual display for indicating the relative position of the test axis when the eye is within a field of view of the optical alignment system, and teaches using a joystick for manually positioning the test axis or using three electric motors controlled by information obtained from the optical alignment system to automatically position the instrument.